"Being is desirable because it is identical with Beauty, and Beauty is loved because it is Being. We ourselves possess Beauty when we are true to our own being; ugliness is in going over to another order; knowing ourselves, we are beautiful; in self-ignorance, we are ugly." -Plotinus

A major problem with a thorough-going holistic monism like Plotinus' is that is begs the question: what is the opposite of being? In Plotinus' view of the world, meditation brings us closer to the One. Physical matter is less permeated with the One than mental entities.

In reply, any positivist will say, as oppossed to what? What else is there other than the One? If the One is all there is, how can something be less permeated with the One than something else? What is the background behind which the One exists? Basically, according to logical positivists, if you say that everything is made of something you aren't saying anything at all.

The response is that this is not a metaphysical theory, but instead, a theory of consciousness, and that, in any case, there is no difference between the two types of theories.

Another response is that there is a rhetorical component to Plotinus' words that is more substantial than the literal meaning. But Plotinus definitely seemed to believe in his system.

And, you know, materialism, for all its contemporary, and hopefully short-lived, hubris, has the same problem. Namely, what is the substance in which protons, neutrons and electrons exist (or quarks, if you prefer the sub-subatomic level)? "A void" is not the most theoretically satisfying of answers.

Others that I consider holistic monists include the Buddha, Lao-Tsu, Heraclitus, Plato, Goethe, Hegel, Schelling, Schopenhaur, Emerson, and Aurobindo. Augustine and Dante were influenced by Plotinus' and Plato's holism. Note that there is an Indian, a Greek, and a German school of holism.